Method and apparatus for encoding video, and method and apparatus for decoding video

ABSTRACT

A method of decoding an image including extracting information that indicates an intra prediction mode applied to a current block to be decoded, from a bitstream; determining one of neighboring pixels adjacent to the current block and filtered neighboring pixels filtered from the neighboring pixels as reference pixels, based on at least one of a size of the current block and an intra prediction mode of the current block; performing intra prediction on the current block using the extracted information and the determined reference pixels, wherein, when the current block has a size of N×N, where N is integer, the neighboring pixels include 2N neighboring pixels adjacent to an upper side and an upper right side of the current block and 2N neighboring pixels adjacent to a left side and a below left side of the current block.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This application is a continuation application of U.S. application Ser.No. 14/536,082, filed Nov. 7, 2014, in the U.S. Patent and TrademarkOffice, which is a continuation application of U.S. application Ser. No.14/287,279, filed on May 27, 2014, in the U.S. Patent and TrademarkOffice, which is a continuation of U.S. application Ser. No. 13/964,881,now U.S. Pat. No. 8,787,458, filed on Aug. 12, 2013, in the U.S. Patentand Trademark Office, which is a continuation of U.S. application Ser.No. 13/874,876, now U.S. Pat. No. 8,989,266, filed on May 1, 2013, inthe U.S. Patent and Trademark Office, which is a continuationapplication of U.S. application Ser. No. 13/371,975, now U.S. Pat. No.8,665,950, filed on Feb. 13, 2012, in the U.S. Patent and TrademarkOffice, which is a continuation application of U.S. application Ser. No.12/857,798, now U.S. Pat. No. 8,605,784, filed on Aug. 17, 2010, in theU.S. Patent and Trademark Office, which claims priority from KoreanPatent Application No. 10-2009-0075855, filed on Aug. 17, 2009, in theKorean Intellectual Property Office, the disclosures of which areincorporated herein by reference in their entireties.

BACKGROUND

1. Field

The exemplary embodiments relate to a method and apparatus for encodinga video, and a method and apparatus for decoding a video, capable ofimproving video compression efficiency by performing intra prediction byusing filtered neighboring pixels.

2. Description of the Related Art

In video compression methods such as MPEG-1, MPEG-2, MPEG-4, andH.264/MPEG-4 Advanced Video Coding (AVC), one picture is split intomacroblocks to encode a video. After that, every macroblock is encodedaccording to all encoding modes available in inter prediction and intraprediction, and then one encoding mode is selected according to a bitrate required to encode the macroblock and distortion between theoriginal macroblock and a decoded macroblock, so as to encode themacroblock.

As hardware for reproducing and storing high resolution or high qualityvideo content is being developed and supplied, a need for a video codecfor effectively encoding or decoding the high resolution or high qualityvideo content is increasing. In a conventional video codec, a video isencoded according to a limited prediction mode based on a macroblockhaving a predetermined size.

SUMMARY

The exemplary embodiments provide a method and apparatus for encoding avideo, and a method and apparatus for decoding a video, capable ofimproving video compression efficiency by filtering neighboring pixelsof a current block and performing intra prediction on the current blockby using the filtered neighboring pixels.

According to an aspect of an exemplary embodiment, there is provided avideo encoding method including filtering neighboring pixels of acurrent block to be encoded so as to generate filtered neighboringpixels; selecting the filtered neighboring pixels or the originalneighboring pixels as reference pixels to be used to perform intraprediction on the current block; and performing intra prediction on thecurrent block by using the selected reference pixels.

According to another aspect of the exemplary embodiment, there isprovided a video decoding method including filtering neighboring pixelsof a current block to be decoded so as to generate filtered neighboringpixels; extracting information about an intra prediction mode applied tothe current block from a bitstream; selecting the filtered neighboringpixels or the original neighboring pixels as reference pixels to be usedto perform intra prediction on the current block; and performing intraprediction on the current block by using the extracted information aboutthe intra prediction mode and the selected reference pixels.

According to another aspect of an exemplary embodiment, there isprovided a video encoding apparatus including a neighboring pixelfiltering unit for filtering neighboring pixels of a current block to beencoded so as to generate filtered neighboring pixels; a reference pixeldetermining unit for selecting the filtered neighboring pixels or theoriginal neighboring pixels as reference pixels to be used to performintra prediction on the current block; and an intra predictionperforming unit for performing intra prediction on the current block byusing the selected reference pixels.

According to another aspect of an exemplary embodiment, there isprovided a video decoding apparatus including a neighboring pixelfiltering unit for filtering neighboring pixels of a current block to bedecoded so as to generate filtered neighboring pixels; an entropydecoder for extracting information about an intra prediction modeapplied to the current block from a bitstream; a reference pixeldetermining unit for selecting the filtered neighboring pixels or theoriginal neighboring pixels as reference pixels to be used to performintra prediction on the current block; and an intra predictionperforming unit for performing intra prediction on the current block byusing the extracted information about the intra prediction mode and theselected reference pixels.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features of the exemplary embodiment will becomemore apparent by describing in detail exemplary embodiments thereof withreference to the attached drawings in which:

FIG. 1 is a block diagram of an apparatus for encoding a video,according to an exemplary embodiment;

FIG. 2 is a block diagram of an apparatus for decoding a video,according to an exemplary embodiment;

FIG. 3 is a diagram for describing a concept of coding units accordingto an exemplary embodiment;

FIG. 4 is a block diagram of an image encoder based on coding unitsaccording to an exemplary embodiment;

FIG. 5 is a block diagram of an image decoder based on coding unitsaccording to an exemplary embodiment;

FIG. 6 is a diagram illustrating deeper coding units according todepths, and partitions according to an exemplary embodiment

FIG. 7 is a diagram for describing a relationship between a coding unitand transform units, according to an exemplary embodiment;

FIG. 8 is a diagram for describing encoding information of coding unitscorresponding to a coded depth, according to an exemplary embodiment;

FIG. 9 is a diagram of deeper coding units according to depths,according to an exemplary embodiment;

FIGS. 10 through 12 are diagrams for describing a relationship betweencoding units, prediction units, and transform units, according to anexemplary embodiment;

FIG. 13 is a diagram for describing a relationship between a codingunit, a prediction unit or a partition, and a transform unit, accordingto encoding mode information of Table 1;

FIG. 14 is a block diagram of an intra prediction apparatus according toan exemplary embodiment;

FIG. 15 is a table showing the numbers of intra prediction modesaccording to the sizes of coding units, according to an exemplaryembodiment;

FIGS. 16A through 16C illustrate intra prediction modes applied to acoding unit having a predetermined size, according to an exemplaryembodiment;

FIG. 17 illustrates intra prediction modes applied to a coding unithaving a predetermined size, according to another exemplary embodiment;

FIGS. 18A through 18C are diagrams for describing intra prediction modeshaving various directivities, according to an exemplary embodiment;

FIG. 19 is a diagram illustrating a current coding unit and neighboringpixels to be filtered, according to an exemplary embodiment;

FIG. 20 is a diagram for describing a process of filtering neighboringpixels, according to an exemplary embodiment;

FIG. 21 is a flowchart illustrating a video encoding method according toan exemplary embodiment; and

FIG. 22 is a flowchart illustrating a video decoding method according toan exemplary embodiment.

DETAILED DESCRIPTION

Hereinafter, the exemplary embodiments will be described more fully withreference to the accompanying drawings, in which exemplary embodimentsare shown. In the exemplary embodiments, “unit” may or may not refer toa unit of size, depending on its context. In the present specification,an “image” may denote a still image for a video or a moving image, thatis, the video itself.

Hereinafter, a ‘coding unit’ is an encoding data unit in which the imagedata is encoded at an encoder side and an encoded data unit in which theencoded image data is decoded at a decoder side, according to exemplaryembodiments. Also, a ‘coded depth’ means a depth where a coding unit isencoded.

Firstly, a method and apparatus for encoding video and a method andapparatus for decoding video, according to an exemplary embodiment, willbe described with reference to FIGS. 1 to 13.

FIG. 1 is a block diagram of a video encoding apparatus 100, accordingto an exemplary embodiment.

The video encoding apparatus 100 includes a maximum coding unit splitter110, a coding unit determiner 120, and an output unit 130.

The maximum coding unit splitter 110 may split a current picture basedon a maximum coding unit for the current picture of an image. If thecurrent picture is larger than the maximum coding unit, image data ofthe current picture may be split into the at least one maximum codingunit. The maximum coding unit according to an exemplary embodiment maybe a data unit having a size of 32×32, 64×64, 128×128, 256×256, etc.,wherein a shape of the data unit is a square having a width and heightin squares of 2. The image data may be output to the coding unitdeterminer 120 according to the at least one maximum coding unit.

A coding unit according to an exemplary embodiment may be characterizedby a maximum size and a depth. The depth denotes a number of times thecoding unit is spatially split from the maximum coding unit, and as thedepth deepens or increases, deeper encoding units according to depthsmay be split from the maximum coding unit to a minimum coding unit. Adepth of the maximum coding unit is an uppermost depth and a depth ofthe minimum coding unit is a lowermost depth. Since a size of a codingunit corresponding to each depth decreases as the depth of the maximumcoding unit deepens, a coding unit corresponding to an upper depth mayinclude a plurality of coding units corresponding to lower depths.

As described above, the image data of the current picture is split intothe maximum coding units according to a maximum size of the coding unit,and each of the maximum coding units may include deeper coding unitsthat are split according to depths. Since the maximum coding unitaccording to an exemplary embodiment is split according to depths, theimage data of a spatial domain included in the maximum coding unit maybe hierarchically classified according to depths.

A maximum depth and a maximum size of a coding unit, which limit thetotal number of times a height and a width of the maximum coding unitare hierarchically split may be predetermined.

The coding unit determiner 120 encodes at least one split regionobtained by splitting a region of the maximum coding unit according todepths, and determines a depth to output a finally encoded image dataaccording to the at least one split region. In other words, the codingunit determiner 120 determines a coded depth by encoding the image datain the deeper coding units according to depths, according to the maximumcoding unit of the current picture, and selects a depth having the leastencoding error. Thus, the encoded image data of the coding unitcorresponding to the determined coded depth is finally output. Also, thecoding units corresponding to the coded depth may be regarded as encodedcoding units.

The determined coded depth and the encoded image data according to thedetermined coded depth are output to the output unit 130.

The image data in the maximum coding unit is encoded based on the deepercoding units corresponding to at least one depth equal to or below themaximum depth, and results of encoding the image data are compared basedon each of the deeper coding units. A depth having the least encodingerror may be selected after comparing encoding errors of the deepercoding units. At least one coded depth may be selected for each maximumcoding unit.

The size of the maximum coding unit is split as a coding unit ishierarchically split according to depths, and as the number of codingunits increases. Also, even if coding units correspond to same depth inone maximum coding unit, it is determined whether to split each of thecoding units corresponding to the same depth to a lower depth bymeasuring an encoding error of the image data of the each coding unit,separately. Accordingly, even when image data is included in one maximumcoding unit, the image data is split to regions according to the depthsand the encoding errors may differ according to regions in the onemaximum coding unit, and thus the coded depths may differ according toregions in the image data. Thus, one or more coded depths may bedetermined in one maximum coding unit, and the image data of the maximumcoding unit may be divided according to coding units of at least onecoded depth.

Accordingly, the coding unit determiner 120 may determine coding unitshaving a tree structure included in the maximum coding unit. The ‘codingunits having a tree structure’ according to an exemplary embodimentinclude coding units corresponding to a depth determined to be the codeddepth, from among all deeper coding units included in the maximum codingunit. A coding unit of a coded depth may be hierarchically determinedaccording to depths in the same region of the maximum coding unit, andmay be independently determined in different regions. Similarly, a codeddepth in a current region may be independently determined from a codeddepth in another region.

A maximum depth according to an exemplary embodiment is an index relatedto the number of splitting times from a maximum coding unit to a minimumcoding unit. A first maximum depth according to an exemplary embodimentmay denote the total number of splitting times from the maximum codingunit to the minimum coding unit. A second maximum depth according to anexemplary embodiment may denote the total number of depth levels fromthe maximum coding unit to the minimum coding unit. For example, when adepth of the maximum coding unit is 0, a depth of a coding unit, inwhich the maximum coding unit is split once, may be set to 1, and adepth of a coding unit, in which the maximum coding unit is split twice,may be set to 2. Here, if the minimum coding unit is a coding unit inwhich the maximum coding unit is split four times, 5 depth levels ofdepths 0, 1, 2, 3 and 4 exist, and thus the first maximum depth may beset to 4, and the second maximum depth may be set to 5.

Prediction encoding and transformation may be performed according to themaximum coding unit. The prediction encoding and the transformation arealso performed based on the deeper coding units according to a depthequal to or depths less than the maximum depth, according to the maximumcoding unit. Transformation may be performed according to method oforthogonal transformation or integer transformation.

Since the number of deeper coding units increases whenever the maximumcoding unit is split according to depths, encoding including theprediction encoding and the transformation is performed on all of thedeeper coding units generated as the depth deepens. For convenience ofdescription, the prediction encoding and the transformation will now bedescribed based on a coding unit of a current depth, in a maximum codingunit.

The video encoding apparatus 100 may variably select a size or shape ofa data unit for encoding the image data. In order to encode the imagedata, operations, such as prediction encoding, transformation, andentropy encoding, are performed, and at this time, the same data unitmay be used for all operations or different data units may be used foreach operation.

For example, the video encoding apparatus 100 may select not only acoding unit for encoding the image data, but also a data unit differentfrom the coding unit so as to perform the prediction encoding on theimage data in the coding unit.

In order to perform prediction encoding in the maximum coding unit, theprediction encoding may be performed based on a coding unitcorresponding to a coded depth, i.e., based on a coding unit that is nolonger split to coding units corresponding to a lower depth.Hereinafter, the coding unit that is no longer split and becomes a basisunit for prediction encoding will now be referred to as a ‘predictionunit’. A partition obtained by splitting the prediction unit may includea prediction unit or a data unit obtained by splitting at least one of aheight and a width of the prediction unit.

For example, when a coding unit of 2N×2N (where N is a positive integer)is no longer split and becomes a prediction unit of 2N×2N, and a size ofa partition may be 2N×2N, 2N×N, N×2N, or N×N. Examples of a partitiontype include symmetrical partitions that are obtained by symmetricallysplitting a height or width of the prediction unit, partitions obtainedby asymmetrically splitting the height or width of the prediction unit,such as 1:n or n:1, partitions that are obtained by geometricallysplitting the prediction unit, and partitions having arbitrary shapes.

A prediction mode of the prediction unit may be at least one of an intramode, a inter mode, and a skip mode. For example, the intra mode or theinter mode may be performed on the partition of 2N×2N, 2N×N, N×2N, orN×N. Also, the skip mode may be performed only on the partition of2N×2N. The encoding is independently performed on one prediction unit ina coding unit, thereby selecting a prediction mode having a leastencoding error.

The video encoding apparatus 100 may also perform the transformation onthe image data in a coding unit based not only on the coding unit forencoding the image data, but also based on a data unit that is differentfrom the coding unit.

In order to perform the transformation in the coding unit, thetransformation may be performed based on a data unit having a sizesmaller than or equal to the coding unit. For example, the data unit forthe transformation may include a data unit for an intra mode and a dataunit for an inter mode.

A data unit used as a base of the transformation will now be referred toas a ‘transform unit’. A transformation depth indicating the number ofsplitting times to reach the transform unit by splitting the height andwidth of the coding unit may also be set in the transform unit. Forexample, in a current coding unit of 2N×2N, a transformation depth maybe 0 when the size of a transform unit is also 2N×2N, may be 1 when eachof the height and width of the current coding unit is split into twoequal parts, totally split into 4^1 transform units, and the size of thetransform unit is thus N×N, and may be 2 when each of the height andwidth of the current coding unit is split into four equal parts, totallysplit into 4^2 transform units and the size of the transform unit isthus N/2×N/2. For example, the transform unit may be set according to ahierarchical tree structure, in which a transform unit of an uppertransformation depth is split into four transform units of a lowertransformation depth according to the hierarchical characteristics of atransformation depth.

Similar to the coding unit, the transform unit in the coding unit may berecursively split into smaller sized regions, so that the transform unitmay be determined independently in units of regions. Thus, residual datain the coding unit may be divided according to the transformation havingthe tree structure according to transformation depths.

Encoding information according to coding units corresponding to a codeddepth requires not only information about the coded depth, but alsoabout information related to prediction encoding and transformation.Accordingly, the coding unit determiner 120 not only determines a codeddepth having a least encoding error, but also determines a partitiontype in a prediction unit, a prediction mode according to predictionunits, and a size of a transform unit for transformation.

Coding units according to a tree structure in a maximum coding unit anda method of determining a partition, according to exemplary embodiments,will be described in detail later with reference to FIGS. 3 through 12.

The coding unit determiner 120 may measure an encoding error of deepercoding units according to depths by using Rate-Distortion Optimizationbased on Lagrangian multipliers.

The output unit 130 outputs the image data of the maximum coding unit,which is encoded based on the at least one coded depth determined by thecoding unit determiner 120, and information about the encoding modeaccording to the coded depth, in bitstreams.

The encoded image data may be obtained by encoding residual data of animage.

The information about the encoding mode according to coded depth mayinclude information about the coded depth, about the partition type inthe prediction unit, the prediction mode, and the size of the transformunit.

The information about the coded depth may be defined by using splitinformation according to depths, which indicates whether encoding isperformed on coding units of a lower depth instead of a current depth.If the current depth of the current coding unit is the coded depth,image data in the current coding unit is encoded and output, and thusthe split information may be defined not to split the current codingunit to a lower depth. Alternatively, if the current depth of thecurrent coding unit is not the coded depth, the encoding is performed onthe coding unit of the lower depth, and thus the split information maybe defined to split the current coding unit to obtain the coding unitsof the lower depth.

If the current depth is not the coded depth, encoding is performed onthe coding unit that is split into the coding unit of the lower depth.Since at least one coding unit of the lower depth exists in one codingunit of the current depth, the encoding is repeatedly performed on eachcoding unit of the lower depth, and thus the encoding may be recursivelyperformed for the coding units having the same depth.

Since the coding units having a tree structure are determined for onemaximum coding unit, and information about at least one encoding mode isdetermined for a coding unit of a coded depth, information about atleast one encoding mode may be determined for one maximum coding unit.Also, a coded depth of the image data of the maximum coding unit may bedifferent according to locations since the image data is hierarchicallysplit according to depths, and thus information about the coded depthand the encoding mode may be set for the image data.

Accordingly, the output unit 130 may assign encoding information about acorresponding coded depth and an encoding mode to at least one of thecoding unit, the prediction unit, and a minimum unit included in themaximum coding unit.

The minimum unit according to an exemplary embodiment is a rectangulardata unit obtained by splitting the minimum coding unit constituting thelowermost depth by 4. Alternatively, the minimum unit may be a maximumrectangular data unit that may be included in all of the coding units,prediction units, partition units, and transform units included in themaximum coding unit.

For example, the encoding information output through the output unit 130may be classified into encoding information according to coding units,and encoding information according to prediction units. The encodinginformation according to the coding units may include the informationabout the prediction mode and about the size of the partitions. Theencoding information according to the prediction units may includeinformation about an estimated direction of an inter mode, about areference image index of the inter mode, about a motion vector, about achroma component of an intra mode, and about an interpolation method ofthe intra mode. Also, information about a maximum size of the codingunit defined according to pictures, slices, or GOPs, and informationabout a maximum depth may be inserted into SPS (Sequence Parameter Set)or a header of a bitstream.

In the video encoding apparatus 100, the deeper coding unit may be acoding unit obtained by dividing a height or width of a coding unit ofan upper depth, which is one layer above, by two. In other words, whenthe size of the coding unit of the current depth is 2N×2N, the size ofthe coding unit of the lower depth is N×N. Also, the coding unit of thecurrent depth having the size of 2N×2N may include maximum 4 of thecoding unit of the lower depth.

Accordingly, the video encoding apparatus 100 may form the coding unitshaving the tree structure by determining coding units having an optimumshape and an optimum size for each maximum coding unit, based on thesize of the maximum coding unit and the maximum depth determinedconsidering characteristics of the current picture. Also, since encodingmay be performed on each maximum coding unit by using any one of variousprediction modes and transformations, an optimum encoding mode may bedetermined considering characteristics of the coding unit of variousimage sizes.

Thus, if an image having high resolution or large data amount is encodedin a conventional macroblock, a number of macroblocks per pictureexcessively increases. Accordingly, a number of pieces of compressedinformation generated for each macroblock increases, and thus it isdifficult to transmit the compressed information and data compressionefficiency decreases. However, by using the video encoding apparatus100, image compression efficiency may be increased since a coding unitis adjusted while considering characteristics of an image whileincreasing a maximum size of a coding unit while considering a size ofthe image.

FIG. 2 is a block diagram of a video decoding apparatus 200, accordingto an exemplary embodiment.

The video decoding apparatus 200 includes a receiver 210, an image dataand encoding information extractor 220, and an image data decoder 230.Definitions of various terms, such as a coding unit, a depth, aprediction unit, a transform unit, and information about variousencoding modes, for various operations of the video decoding apparatus200 are identical to those described with reference to FIG. 1 and thevideo encoding apparatus 100.

The receiver 210 receives and parses a bitstream of an encoded video.The image data and encoding information extractor 220 extracts encodedimage data for each coding unit from the parsed bitstream, wherein thecoding units have a tree structure according to each maximum codingunit, and outputs the extracted image data to the image data decoder230. The image data and encoding information extractor 220 may extractinformation about a maximum size of a coding unit of a current picture,from a header about the current picture or SPS.

Also, the image data and encoding information extractor 220 extractsinformation about a coded depth and an encoding mode for the codingunits having a tree structure according to each maximum coding unit,from the parsed bitstream. The extracted information about the codeddepth and the encoding mode is output to the image data decoder 230. Inother words, the image data in a bit stream is split into the maximumcoding unit so that the image data decoder 230 decodes the image datafor each maximum coding unit.

The information about the coded depth and the encoding mode according tothe maximum coding unit may be set for information about at least onecoding unit corresponding to the coded depth, and information about anencoding mode may include information about a partition type of acorresponding coding unit corresponding to the coded depth, about aprediction mode, and a size of a transform unit. Also, splittinginformation according to depths may be extracted as the informationabout the coded depth.

The information about the coded depth and the encoding mode according toeach maximum coding unit extracted by the image data and encodinginformation extractor 220 is information about a coded depth and anencoding mode determined to generate a minimum encoding error when anencoder, such as the video encoding apparatus 100, repeatedly performsencoding for each deeper coding unit according to depths according toeach maximum coding unit. Accordingly, the video decoding apparatus 200may restore an image by decoding the image data according to a codeddepth and an encoding mode that generates the minimum encoding error.

Since encoding information about the coded depth and the encoding modemay be assigned to a predetermined data unit from among a correspondingcoding unit, a prediction unit, and a minimum unit, the image data andencoding information extractor 220 may extract the information about thecoded depth and the encoding mode according to the predetermined dataunits. The predetermined data units to which the same information aboutthe coded depth and the encoding mode is assigned may be inferred to bethe data units included in the same maximum coding unit.

The image data decoder 230 restores the current picture by decoding theimage data in each maximum coding unit based on the information aboutthe coded depth and the encoding mode according to the maximum codingunits. In other words, the image data decoder 230 may decode the encodedimage data based on the extracted information about the partition type,the prediction mode, and the transform unit for each coding unit fromamong the coding units having the tree structure included in eachmaximum coding unit. A decoding process may include a predictionincluding intra prediction and motion compensation, and an inversetransformation. Inverse transformation may be performed according tomethod of inverse orthogonal transformation or inverse integertransformation.

The image data decoder 230 may perform intra prediction or motioncompensation according to a partition and a prediction mode of eachcoding unit, based on the information about the partition type and theprediction mode of the prediction unit of the coding unit according tocoded depths.

Also, the image data decoder 230 may perform inverse transformationaccording to each transform unit in the coding unit, based on theinformation about the size of the transform unit of the coding unitaccording to coded depths, so as to perform the inverse transformationaccording to maximum coding units.

The image data decoder 230 may determine at least one coded depth of acurrent maximum coding unit by using split information according todepths. If the split information indicates that image data is no longersplit in the current depth, the current depth is a coded depth.Accordingly, the image data decoder 230 may decode encoded data of atleast one coding unit corresponding to the each coded depth in thecurrent maximum coding unit by using the information about the partitiontype of the prediction unit, the prediction mode, and the size of thetransform unit for each coding unit corresponding to the coded depth,and output the image data of the current maximum coding unit.

In other words, data units containing the encoding information includingthe same split information may be gathered by observing the encodinginformation set assigned for the predetermined data unit from among thecoding unit, the prediction unit, and the minimum unit, and the gathereddata units may be considered to be one data unit to be decoded by theimage data decoder 230 in the same encoding mode.

The video decoding apparatus 200 may obtain information about at leastone coding unit that generates the minimum encoding error when encodingis recursively performed for each maximum coding unit, and may use theinformation to decode the current picture. In other words, the codingunits having the tree structure determined to be the optimum codingunits in each maximum coding unit may be decoded. Also, the maximum sizeof coding unit is determined considering resolution and an amount ofimage data.

Accordingly, even if image data has high resolution and a large amountof data, the image data may be efficiently decoded and restored by usinga size of a coding unit and an encoding mode, which are adaptivelydetermined according to characteristics of the image data, by usinginformation about an optimum encoding mode received from an encoder.

A method of determining coding units having a tree structure, aprediction unit, and a transform unit, according to an exemplaryembodiment, will now be described with reference to FIGS. 3 through 13.

FIG. 3 is a diagram for describing a concept of coding units accordingto an exemplary embodiment.

A size of a coding unit may be expressed in width×height, and may be64×64, 32×32, 16×16, and 8×8. A coding unit of 64×64 may be split intopartitions of 64×64, 64×32, 32×64, or 32×32, and a coding unit of 32×32may be split into partitions of 32×32, 32×16, 16×32, or 16×16, a codingunit of 16×16 may be split into partitions of 16×16, 16×8, 8×16, or 8×8,and a coding unit of 8×8 may be split into partitions of 8×8, 8×4, 4×8,or 4×4.

In video data 310, a resolution is 1920×1080, a maximum size of a codingunit is 64, and a maximum depth is 2. In video data 320, a resolution is1920×1080, a maximum size of a coding unit is 64, and a maximum depth is3. In video data 330, a resolution is 352×288, a maximum size of acoding unit is 16, and a maximum depth is 1. The maximum depth shown inFIG. 3 denotes a total number of splits from a maximum coding unit to aminimum decoding unit.

If a resolution is high or a data amount is large, a maximum size of acoding unit may be large so as to not only increase encoding efficiencybut also to accurately reflect characteristics of an image. Accordingly,the maximum size of the coding unit of the video data 310 and 320 havingthe higher resolution than the video data 330 may be 64.

Since the maximum depth of the video data 310 is 2, coding units 315 ofthe video data 310 may include a maximum coding unit having a long axissize of 64, and coding units having long axis sizes of 32 and 16 sincedepths are deepened to two layers by splitting the maximum coding unittwice. Meanwhile, since the maximum depth of the video data 330 is 1,coding units 335 of the video data 330 may include a maximum coding unithaving a long axis size of 16, and coding units having a long axis sizeof 8 since depths are deepened to one layer by splitting the maximumcoding unit once.

Since the maximum depth of the video data 320 is 3, coding units 325 ofthe video data 320 may include a maximum coding unit having a long axissize of 64, and coding units having long axis sizes of 32, 16, and 8since the depths are deepened to 3 layers by splitting the maximumcoding unit three times. As a depth deepens, detailed information may beprecisely expressed.

FIG. 4 is a block diagram of an image encoder 400 based on coding units,according to an exemplary embodiment.

The image encoder 400 performs operations of the coding unit determiner120 of the video encoding apparatus 100 to encode image data. In otherwords, an intra predictor 410 performs intra prediction on coding unitsin an intra mode, from among a current frame 405, and a motion estimator420 and a motion compensator 425 performs inter estimation and motioncompensation on coding units in an inter mode from among the currentframe 405 by using the current frame 405, and a reference frame 495.

Data output from the intra predictor 410, the motion estimator 420, andthe motion compensator 425 is output as a quantized transformationcoefficient through a transformer 430 and a quantizer 440. The quantizedtransformation coefficient is restored as data in a spatial domainthrough an inverse quantizer 460 and an inverse transformer 470, and therestored data in the spatial domain is output as the reference frame 495after being post-processed through a deblocking unit 480 and a loopfiltering unit 490. The quantized transformation coefficient may beoutput as a bitstream 455 through an entropy encoder 450.

In order for the image encoder 400 to be applied in the video encodingapparatus 100, all elements of the image encoder 400, i.e., the intrapredictor 410, the motion estimator 420, the motion compensator 425, thetransformer 430, the quantizer 440, the entropy encoder 450, the inversequantizer 460, the inverse transformer 470, the deblocking unit 480, andthe loop filtering unit 490 perform operations based on each coding unitfrom among coding units having a tree structure while considering themaximum depth of each maximum coding unit.

Specifically, the intra predictor 410, the motion estimator 420, and themotion compensator 425 determines partitions and a prediction mode ofeach coding unit from among the coding units having a tree structurewhile considering the maximum size and the maximum depth of a currentmaximum coding unit, and the transformer 430 determines the size of thetransform unit in each coding unit from among the coding units having atree structure.

FIG. 5 is a block diagram of an image decoder 500 based on coding units,according to an exemplary embodiment.

A parser 510 parses encoded image data to be decoded and informationabout encoding required for decoding from a bitstream 505. The encodedimage data is output as inverse quantized data through an entropydecoder 520 and an inverse quantizer 530, and the inverse quantized datais restored to image data in a spatial domain through an inversetransformer 540.

An intra predictor 550 performs intra prediction on coding units in anintra mode with respect to the image data in the spatial domain, and amotion compensator 560 performs motion compensation on coding units inan inter mode by using a reference frame 585.

The image data in the spatial domain, which passed through the intrapredictor 550 and the motion compensator 560, may be output as arestored frame 595 after being post-processed through a deblocking unit570 and a loop filtering unit 580. Also, the image data that ispost-processed through the deblocking unit 570 and the loop filteringunit 580 may be output as the reference frame 585.

In order to decode the image data in the image data decoder 230 of thevideo decoding apparatus 200, the image decoder 500 may performoperations that are performed after the parser 510.

In order for the image decoder 500 to be applied in the video decodingapparatus 200, all elements of the image decoder 500, i.e., the parser510, the entropy decoder 520, the inverse quantizer 530, the inversetransformer 540, the intra predictor 550, the motion compensator 560,the deblocking unit 570, and the loop filtering unit 580 performoperations based on coding units having a tree structure for eachmaximum coding unit.

Specifically, the intra prediction 550 and the motion compensator 560perform operations based on partitions and a prediction mode for each ofthe coding units having a tree structure, and the inverse transformer540 perform operations based on a size of a transform unit for eachcoding unit.

FIG. 6 is a diagram illustrating deeper coding units according todepths, and partitions, according to an exemplary embodiment.

The video encoding apparatus 100 and the video decoding apparatus 200use hierarchical coding units so as to consider characteristics of animage. A maximum height, a maximum width, and a maximum depth of codingunits may be adaptively determined according to the characteristics ofthe image, or may be differently set by a user. Sizes of deeper codingunits according to depths may be determined according to thepredetermined maximum size of the coding unit.

In a hierarchical structure 600 of coding units, according to anexemplary embodiment, the maximum height and the maximum width of thecoding units are each 64, and the maximum depth is 4. Since a depthdeepens along a vertical axis of the hierarchical structure 600, aheight and a width of the deeper coding unit are each split. Also, aprediction unit and partitions, which are bases for prediction encodingof each deeper coding unit, are shown along a horizontal axis of thehierarchical structure 600.

In other words, a coding unit 610 is a maximum coding unit in thehierarchical structure 600, wherein a depth is 0 and a size, i.e., aheight by width, is 64×64. The depth deepens along the vertical axis,and a coding unit 620 having a size of 32×32 and a depth of 1, a codingunit 630 having a size of 16×16 and a depth of 2, a coding unit 640having a size of 8×8 and a depth of 3, and a coding unit 650 having asize of 4×4 and a depth of 4 exist. The coding unit 650 having the sizeof 4×4 and the depth of 4 is a minimum coding unit.

The prediction unit and the partitions of a coding unit are arrangedalong the horizontal axis according to each depth. In other words, ifthe coding unit 610 having the size of 64×64 and the depth of 0 is aprediction unit, the prediction unit may be split into partitionsinclude in the encoding unit 610, i.e. a partition 610 having a size of64×64, partitions 612 having the size of 64×32, partitions 614 havingthe size of 32×64, or partitions 616 having the size of 32×32.

Similarly, a prediction unit of the coding unit 620 having the size of32×32 and the depth of 1 may be split into partitions included in thecoding unit 620, i.e. a partition 620 having a size of 32×32, partitions622 having a size of 32×16, partitions 624 having a size of 16×32, andpartitions 626 having a size of 16×16.

Similarly, a prediction unit of the coding unit 630 having the size of16×16 and the depth of 2 may be split into partitions included in thecoding unit 630, i.e. a partition having a size of 16×16 included in thecoding unit 630, partitions 632 having a size of 16×8, partitions 634having a size of 8×16, and partitions 636 having a size of 8×8.

Similarly, a prediction unit of the coding unit 640 having the size of8×8 and the depth of 3 may be split into partitions included in thecoding unit 640, i.e. a partition having a size of 8×8 included in thecoding unit 640, partitions 642 having a size of 8×4, partitions 644having a size of 4×8, and partitions 646 having a size of 4×4.

The coding unit 650 having the size of 4×4 and the depth of 4 is theminimum coding unit and a coding unit of the lowermost depth. Aprediction unit of the coding unit 650 is only assigned to a partitionhaving a size of 4×4.

In order to determine the at least one coded depth of the coding unitsconstituting the maximum coding unit 610, the coding unit determiner 120of the video encoding apparatus 100 performs encoding for coding unitscorresponding to each depth included in the maximum coding unit 610.

A number of deeper coding units according to depths including data inthe same range and the same size increases as the depth deepens. Forexample, four coding units corresponding to a depth of 2 are required tocover data that is included in one coding unit corresponding to a depthof 1. Accordingly, in order to compare encoding results of the same dataaccording to depths, the coding unit corresponding to the depth of 1 andfour coding units corresponding to the depth of 2 are each encoded.

In order to perform encoding for a current depth from among the depths,a least encoding error may be selected for the current depth byperforming encoding for each prediction unit in the coding unitscorresponding to the current depth, along the horizontal axis of thehierarchical structure 600. Alternatively, the minimum encoding errormay be searched for by comparing the least encoding errors according todepths, by performing encoding for each depth as the depth deepens alongthe vertical axis of the hierarchical structure 600. A depth and apartition having the minimum encoding error in the coding unit 610 maybe selected as the coded depth and a partition type of the coding unit610.

FIG. 7 is a diagram for describing a relationship between a coding unit710 and transform units 720, according to an exemplary embodiment.

The video encoding apparatus 100 or 200 encodes or decodes an imageaccording to coding units having sizes smaller than or equal to amaximum coding unit for each maximum coding unit. Sizes of transformunits for transformation during encoding may be selected based on dataunits that are not larger than a corresponding coding unit.

For example, in the video encoding apparatus 100 or 200, if a size ofthe coding unit 710 is 64×64, transformation may be performed by usingthe transform units 720 having a size of 32×32.

Also, data of the coding unit 710 having the size of 64×64 may beencoded by performing the transformation on each of the transform unitshaving the size of 32×32, 16×16, 8×8, and 4×4, which are smaller than64×64, and then a transform unit having the least coding error may beselected.

FIG. 8 is a diagram for describing encoding information of coding unitscorresponding to a coded depth, according to an exemplary embodiment.

The output unit 130 of the video encoding apparatus 100 may encode andtransmit information 800 about a partition type, information 810 about aprediction mode, and information 820 about a size of a transform unitfor each coding unit corresponding to a coded depth, as informationabout an encoding mode.

The information 800 indicates information about a shape of a partitionobtained by splitting a prediction unit of a current coding unit,wherein the partition is a data unit for prediction encoding the currentcoding unit. For example, a current coding unit CU_0 having a size of2N×2N may be split into any one of a partition 802 having a size of2N×2N, a partition 804 having a size of 2N×N, a partition 806 having asize of N×2N, and a partition 808 having a size of N×N. Here, theinformation 800 about a partition type is set to indicate one of thepartition 804 having a size of 2N×N, the partition 806 having a size ofN×2N, and the partition 808 having a size of N×N

The information 810 indicates a prediction mode of each partition. Forexample, the information 810 may indicate a mode of prediction encodingperformed on a partition indicated by the information 800, i.e., anintra mode 812, an inter mode 814, or a skip mode 816.

The information 820 indicates a transform unit to be based on whentransformation is performed on a current coding unit. For example, thetransform unit may be a first intra transform unit 822, a second intratransform unit 824, a first inter transform unit 826, or a second intratransform unit 828.

The image data and encoding information extractor 220 of the videodecoding apparatus 200 may extract and use the information 800, 810, and820 for decoding, according to each deeper coding unit

FIG. 9 is a diagram of deeper coding units according to depths,according to an exemplary embodiment.

Split information may be used to indicate a change of a depth. The spiltinformation indicates whether a coding unit of a current depth is splitinto coding units of a lower depth.

A prediction unit 910 for prediction encoding a coding unit 900 having adepth of 0 and a size of 2N_0×2N_0 may include partitions of a partitiontype 912 having a size of 2N_0×2N_0, a partition type 914 having a sizeof 2N_0×N_0, a partition type 916 having a size of N_0×2N_0, and apartition type 918 having a size of N_0×N_0. FIG. 9 only illustrates thepartition types 912 through 918 which are obtained by symmetricallysplitting the prediction unit 910, but a partition type is not limitedthereto, and the partitions of the prediction unit 910 may includeasymmetrical partitions, partitions having a predetermined shape, andpartitions having a geometrical shape.

Prediction encoding is repeatedly performed on one partition having asize of 2N_0×2N_0, two partitions having a size of 2N_0×N_0, twopartitions having a size of N_0×2N_0, and four partitions having a sizeof N_0×N_0, according to each partition type. The prediction encoding inan intra mode and an inter mode may be performed on the partitionshaving the sizes of 2N_0×2N_0, N_0×2N_0, 2N_0×N_0, and N_0×N_0. Theprediction encoding in a skip mode is performed only on the partitionhaving the size of 2N_0×2N_0.

Errors of encoding including the prediction encoding in the partitiontypes 912 through 918 are compared, and the least encoding error isdetermined among the partition types. If an encoding error is smallestin one of the partition types 912 through 916, the prediction unit 910may not be split into a lower depth.

If the encoding error is the smallest in the partition type 918, a depthis changed from 0 to 1 to split the partition type 918 in operation 920,and encoding is repeatedly performed on coding units 930 having a depthof 2 and a size of N_0×N_0 to search for a minimum encoding error.

A prediction unit 940 for prediction encoding the coding unit 930 havinga depth of 1 and a size of 2N_1×2N_1 (=N_0×N_0) may include partitionsof a partition type 942 having a size of 2N_1×2N_1, a partition type 944having a size of 2N_1×N_1, a partition type 946 having a size ofN_1×2N_1, and a partition type 948 having a size of N_1×N_1.

If an encoding error is the smallest in the partition type 948, a depthis changed from 1 to 2 to split the partition type 948 in operation 950,and encoding is repeatedly performed on coding units 960, which have adepth of 2 and a size of N_2×N_2 to search for a minimum encoding error.

When a maximum depth is d, split operation according to each depth maybe performed up to when a depth becomes d−1, and split information maybe encoded as up to when a depth is one of 0 to d−2. In other words,when encoding is performed up to when the depth is d−1 after a codingunit corresponding to a depth of d−2 is split in operation 970, aprediction unit 990 for prediction encoding a coding unit 980 having adepth of d−1 and a size of 2N_(d−1)×2N_(d−1) may include partitions of apartition type 992 having a size of 2N_(d−1)×2N_(d−1), a partition type994 having a size of 2N_(d−1)×N_(d−1), a partition type 996 having asize of N_(d−1)×2N_(d−1), and a partition type 998 having a size ofN_(d−1)×N_(d−1).

Prediction encoding may be repeatedly performed on one partition havinga size of 2N_(d−1)×2N_(d−1), two partitions having a size of2N_(d−1)×N_(d−1), two partitions having a size of N_(d−1)×2N_(d−1), fourpartitions having a size of N_(d−1)×N_(d−1) from among the partitiontypes 992 through 998 to search for a partition type having a minimumencoding error.

Even when the partition type 998 has the minimum encoding error, since amaximum depth is d, a coding unit CU_(d−1) having a depth of d−1 is nolonger split to a lower depth, and a coded depth for the coding unitsconstituting a current maximum coding unit 900 is determined to be d−1and a partition type of the current maximum coding unit 900 may bedetermined to be N_(d−1)×N_(d−1). Also, since the maximum depth is d anda minimum coding unit 980 having a lowermost depth of d−1 is no longersplit to a lower depth, split information for the minimum coding unit980 is not set.

A data unit 999 may be a ‘minimum unit’ for the current maximum codingunit. A minimum unit according to an exemplary embodiment may be arectangular data unit obtained by splitting a minimum coding unit 980 by4. By performing the encoding repeatedly, the video encoding apparatus100 may select a depth having the least encoding error by comparingencoding errors according to depths of the coding unit 900 to determinea coded depth, and set a corresponding partition type and a predictionmode as an encoding mode of the coded depth.

As such, the minimum encoding errors according to depths are compared inall of the depths of 1 through d, and a depth having the least encodingerror may be determined as a coded depth. The coded depth, the partitiontype of the prediction unit, and the prediction mode may be encoded andtransmitted as information about an encoding mode. Also, since a codingunit is split from a depth of 0 to a coded depth, only split informationof the coded depth is set to 0, and split information of depthsexcluding the coded depth is set to 1.

The image data and encoding information extractor 220 of the videodecoding apparatus 200 may extract and use the information about thecoded depth and the prediction unit of the coding unit 900 to decode thepartition 912. The video decoding apparatus 200 may determine a depth,in which split information is 0, as a coded depth by using splitinformation according to depths, and use information about an encodingmode of the corresponding depth for decoding.

FIGS. 10 through 12 are diagrams for describing a relationship betweencoding units 1010, prediction units 1060, and transform units 1070,according to an exemplary embodiment.

The coding units 1010 are coding units having a tree structure,corresponding to coded depths determined by the video encoding apparatus100, in a maximum coding unit. The prediction units 1060 are partitionsof prediction units of each of the coding units 1010, and the transformunits 1070 are transform units of each of the coding units 1010.

When a depth of a maximum coding unit is 0 in the coding units 1010,depths of coding units 1012 and 1054 are 1, depths of coding units 1014,1016, 1018, 1028, 1050, and 1052 are 2, depths of coding units 1020,1022, 1024, 1026, 1030, 1032, and 1048 are 3, and depths of coding units1040, 1042, 1044, and 1046 are 4.

In the prediction units 1060, some encoding units 1014, 1016, 1022,1032, 1048, 1050, 1052, and 1054 are obtained by splitting the codingunits in the encoding units 1010. In other words, partition types in thecoding units 1014, 1022, 1050, and 1054 have a size of 2N×N, partitiontypes in the coding units 1016, 1048, and 1052 have a size of N×2N, anda partition type of the coding unit 1032 has a size of N×N. Predictionunits and partitions of the coding units 1010 are smaller than or equalto each coding unit.

Transformation or inverse transformation is performed on image data ofthe coding unit 1052 in the transform units 1070 in a data unit that issmaller than the coding unit 1052. Also, the coding units 1014, 1016,1022, 1032, 1048, 1050, and 1052 in the transform units 1070 aredifferent from those in the prediction units 1060 in terms of sizes andshapes. In other words, the video encoding and decoding apparatuses 100and 200 may perform intra prediction, motion estimation, motioncompensation, transformation, and inverse transformation individually ona data unit in the same coding unit.

Accordingly, encoding is recursively performed on each of coding unitshaving a hierarchical structure in each region of a maximum coding unitto determine an optimum coding unit, and thus coding units having arecursive tree structure may be obtained. Encoding information mayinclude split information about a coding unit, information about apartition type, information about a prediction mode, and informationabout a size of a transform unit. Table 1 shows the encoding informationthat may be set by the video encoding and decoding apparatuses 100 and200.

TABLE 1 Split Information 0 Split (Encoding on Coding Unit having Sizeof 2N × 2N and Current Depth of d) Information 1 Prediction PartitionType Size of Transform unit Repeatedly Mode Encode Intra SymmetricalAsymmetrical Split Split Coding Units Inter Partition PartitionInformation 0 of Information 1 of having Skip Type Type Transform unitTransform unit Lower Depth (Only 2N × 2N 2N × nU 2N × 2N N × N of d + 12N × 2N) 2N × N 2N × nD (Symmetrical Type) N × 2N nL × 2N N/2 × N/2 N ×N nR × 2N (Asymmetrical Type)

The output unit 130 of the video encoding apparatus 100 may output theencoding information about the coding units having a tree structure, andthe image data and encoding information extractor 220 of the videodecoding apparatus 200 may extract the encoding information about thecoding units having a tree structure from a received bitstream.

Split information indicates whether a current coding unit is split intocoding units of a lower depth. If split information of a current depth dis 0, a depth, in which a current coding unit is no longer split into alower depth, is a coded depth, and thus information about a partitiontype, prediction mode, and a size of a transform unit may be defined forthe coded depth. If the current coding unit is further split accordingto the split information, encoding is independently performed on foursplit coding units of a lower depth.

A prediction mode may be one of an intra mode, an inter mode, and a skipmode. The intra mode and the inter mode may be defined in all partitiontypes, and the skip mode is defined only in a partition type having asize of 2N×2N.

The information about the partition type may indicate symmetricalpartition types having sizes of 2N×2N, 2N×N, N×2N, and N×N, which areobtained by symmetrically splitting a height or a width of a predictionunit, and asymmetrical partition types having sizes of 2N×nU, 2N×nD,nL×2N, and nR×2N, which are obtained by asymmetrically splitting theheight or width of the prediction unit. The asymmetrical partition typeshaving the sizes of 2N×nU and 2N×nD may be respectively obtained bysplitting the height of the prediction unit in 1:3 and 3:1, and theasymmetrical partition types having the sizes of nL×2N and nR×2N may berespectively obtained by splitting the width of the prediction unit in1:3 and 3:1

The size of the transform unit may be set to be two types in the intramode and two types in the inter mode. In other words, if splitinformation of the transform unit is 0, the size of the transform unitmay be 2N×2N, which is the size of the current coding unit. If splitinformation of the transform unit is 1, the transform units may beobtained by splitting the current coding unit. Also, if a partition typeof the current coding unit having the size of 2N×2N is a symmetricalpartition type, a size of a transform unit may be N×N, and if thepartition type of the current coding unit is an asymmetrical partitiontype, the size of the transform unit may be N/2×N/2.

The encoding information about coding units having a tree structure mayinclude at least one of a coding unit corresponding to a coded depth, aprediction unit, and a minimum unit. The coding unit corresponding tothe coded depth may include at least one of a prediction unit and aminimum unit containing the same encoding information.

Accordingly, it is determined whether adjacent data units are includedin the same coding unit corresponding to the coded depth by comparingencoding information of the adjacent data units. Also, a correspondingcoding unit corresponding to a coded depth is determined by usingencoding information of a data unit, and thus a distribution of codeddepths in a maximum coding unit may be determined.

Accordingly, if a current coding unit is predicted based on encodinginformation of adjacent data units, encoding information of data unitsin deeper coding units adjacent to the current coding unit may bedirectly referred to and used.

Alternatively, if a current coding unit is predicted based on encodinginformation of adjacent data units, data units adjacent to the currentcoding unit are searched using encoding information of the data units,and the searched adjacent coding units may be referred for predictingthe current coding unit.

FIG. 13 is a diagram for describing a relationship between a codingunit, a prediction unit or a partition, and a transform unit, accordingto encoding mode information of Table 1.

A maximum coding unit 1300 includes coding units 1302, 1304, 1306, 1312,1314, 1316, and 1318 of coded depths. Here, since the coding unit 1318is a coding unit of a coded depth, split information may be set to 0.Information about a partition type of the coding unit 1318 having a sizeof 2N×2N may be set to be one of a partition type 1322 having a size of2N×2N, a partition type 1324 having a size of 2N×N, a partition type1326 having a size of N×2N, a partition type 1328 having a size of N×N,a partition type 1332 having a size of 2N×nU, a partition type 1334having a size of 2N×nD, a partition type 1336 having a size of nL×2N,and a partition type 1338 having a size of nR×2N.

When the partition type is set to be symmetrical, i.e., the partitiontype 1322, 1324, 1326, or 1328, a transform unit 1342 having a size of2N×2N is set if split information (TU size flag) of a transform unit is0, and a transform unit 1344 having a size of N×N is set if a TU sizeflag is 1.

When the partition type is set to be asymmetrical, i.e., the partitiontype 1332, 1334, 1336, or 1338, a transform unit 1352 having a size of2N×2N is set if a TU size flag is 0, and a transform unit 1354 having asize of N/2×N/2 is set if a TU size flag is 1.

Referring to FIG. 13, the TU size flag is a flag having a value or 0 or1, but the TU size flag is not limited to 1 bit, and a transform unitmay be hierarchically split having a tree structure while the TU sizeflag increases from 0.

In this case, the size of a transform unit that has been actually usedmay be expressed by using a TU size flag of a transform unit, accordingto an exemplary embodiment, together with a maximum size and minimumsize of the transform unit. According to an exemplary embodiment, thevideo encoding apparatus 100 is capable of encoding maximum transformunit size information, minimum transform unit size information, and amaximum TU size flag. The result of encoding the maximum transform unitsize information, the minimum transform unit size information, and themaximum TU size flag may be inserted into an SPS. According to anexemplary embodiment, the video decoding apparatus 200 may decode videoby using the maximum transform unit size information, the minimumtransform unit size information, and the maximum TU size flag.

For example, if the size of a current coding unit is 64×64 and a maximumtransform unit size is 32×32, then the size of a transform unit may be32×32 when a TU size flag is 0, may be 16×16 when the TU size flag is 1,and may be 8×8 when the TU size flag is 2.

As another example, if the size of the current coding unit is 32×32 anda minimum transform unit size is 32×32, then the size of the transformunit may be 32×32 when the TU size flag is 0. Here, the TU size flagcannot be set to a value other than 0, since the size of the transformunit cannot be less than 32×32.

As another example, if the size of the current coding unit is 64×64 anda maximum TU size flag is 1, then the TU size flag may be 0 or 1. Here,the TU size flag cannot be set to a value other than 0 or 1.

Thus, if it is defined that the maximum TU size flag is‘MaxTransformSizeIndex’, a minimum transform unit size is‘MinTransformSize’, and a transform unit size is ‘RootTuSize’ when theTU size flag is 0, then a current minimum transform unit size‘CurrMinTuSize’ that can be determined in a current coding unit, may bedefined by Equation (1):CurrMinTuSize=max(MinTransformSize,RootTuSize/(2^MaxTransformSizeIndex))  (1)

Compared to the current minimum transform unit size ‘CurrMinTuSize’ thatcan be determined in the current coding unit, a transform unit size‘RootTuSize’ when the TU size flag is 0 may denote a maximum transformunit size that can be selected in the system. In Equation (1),‘RootTuSize/(2^MaxTransformSizeIndex)’ denotes a transform unit sizewhen the transform unit size ‘RootTuSize’, when the TU size flag is 0,is split a number of times corresponding to the maximum TU size flag,and ‘MinTransformSize’ denotes a minimum transformation size. Thus, asmaller value from among ‘RootTuSize/(2^MaxTransformSizeIndex)’ and‘MinTransformSize’ may be the current minimum transform unit size‘CurrMinTuSize’ that can be determined in the current coding unit.

According to an exemplary embodiment, the maximum transform unit sizeRootTuSize may vary according to the type of a prediction mode.

For example, if a current prediction mode is an inter mode, then‘RootTuSize’ may be determined by using Equation (2) below. In Equation(2), ‘MaxTransformSize’ denotes a maximum transform unit size, and‘PUSize’ denotes a current prediction unit size.RootTuSize=min(MaxTransformSize,PUSize)  (2)

That is, if the current prediction mode is the inter mode, the transformunit size ‘RootTuSize’ when the TU size flag is 0, may be a smallervalue from among the maximum transform unit size and the currentprediction unit size.

If a prediction mode of a current partition unit is an intra mode,‘RootTuSize’ may be determined by using Equation (3) below. In Equation(3), ‘PartitionSize’ denotes the size of the current partition unit.RootTuSize=min(MaxTransformSize,PartitionSize)  (3)

That is, if the current prediction mode is the intra mode, the transformunit size ‘RootTuSize’ when the TU size flag is 0 may be a smaller valuefrom among the maximum transform unit size and the size of the currentpartition unit.

However, the current maximum transform unit size ‘RootTuSize’ thatvaries according to the type of a prediction mode in a partition unit isjust an example and is not limited thereto.

Intra prediction performed by the intra prediction unit 410 of the videoencoding apparatus 100 illustrated in FIG. 4 and the intra predictionunit 550 of the video decoding apparatus 200 illustrated in FIG. 5 willnow be described in detail. In the following description, an encodingunit denotes a current encoded block in an encoding process of an image,and a decoding unit denotes a current decoded block in a decodingprocess of an image. The encoding unit and the decoding unit aredifferent only in that the encoding unit is used in the encoding processand the decoding unit is used in the decoding. For the consistency ofterms, except for a particular case, the encoding unit and the decodingunit are referred to as a coding unit in both the encoding and decodingprocesses. Also, the coding unit may be prediction unit, predictionpartition and block. Also, one of ordinary skill in the art wouldunderstand by the present specification that an intra prediction methodand apparatus according to an exemplary embodiment may also be appliedto perform intra prediction in a general video codec.

FIG. 14 is a block diagram of an intra prediction apparatus 1200according to an exemplary embodiment.

Referring to FIG. 14, the intra prediction apparatus 1200 includes aneighboring pixel filtering unit 1210, a reference pixel determiningunit 1220, and an intra prediction performing unit 1230.

The neighboring pixel filtering unit 1210 filters neighboring pixelsused to perform intra prediction on a current coding unit to be encodedso as to generate filtered neighboring pixels. The filtering of theneighboring pixels will now be described with reference to FIGS. 19 and20.

FIG. 19 is a diagram illustrating a current coding unit 1700 andneighboring pixels 1710 and 1720 to be filtered, according to anexemplary embodiment.

Referring to FIG. 19, the neighboring pixel filtering unit 1210 filtersX neighboring pixels 1710 at an upper side of the current coding unit1700 and Y neighboring pixels 1720 at a left side of the current codingunit 1700 at least once so as to generate filtered neighboring pixels.Here, if the current coding unit 1700 has a size of N×N, the neighboringpixel filtering unit 1210 may filter 4N neighboring pixels such as 2Nneighboring pixels 1710 at the upper side of the current coding unit1700 and 2N neighboring pixels 1720 at the left side of the currentcoding unit 1700. That is, X=2N and Y=2N. The number of the neighboringpixels 1710 and 1720 filtered by the neighboring pixel filtering unit1210 is not limited thereto and may be changed in consideration of thedirectivity of an intra prediction mode applied to the current codingunit 1700.

Also, in FIG. 19, if X+Y original neighboring pixels 1710 and 1720 atthe upper and left sides of the current coding unit 1700 are representedby ContextOrg[n] (where n is an integer from 0 to X+Y−1), and alowermost neighboring pixel of the Y neighboring pixels 1720 has a valueof n=0, i.e., ContextOrg[0], a rightmost neighboring pixel of the Xneighboring pixels 1710 has a value of n=X+Y−1, i.e., ContextOrg[X+Y−1].

FIG. 20 is a diagram for describing a process of filtering neighboringpixels, according to an exemplary embodiment.

Referring to FIG. 20, if 4N original neighboring pixels at upper andleft sides of a current coding unit having a size of N×N are representedby ContextOrg[n] (where n is an integer from 0 to 4N−1), the neighboringpixel filtering unit 1210 filters the original neighboring pixels bycalculating weighted average values between the original neighboringpixels so as to generate first filtered neighboring pixelsContextFiltered1[n]. For example, the neighboring pixel filtering unit1210 generates the first filtered neighboring pixels by applying a 3-tapfilter to the original neighboring pixels ContextOrg[n] as representedby Equation (4).ContextFiltered1[n]=(ContextOrg[n−1]+2*ContextOrg[n]+ContextOrg[n+1])/4  (4)

Referring to Equation (4), the neighboring pixel filtering unit 1210calculates a weighted average value of a neighboring pixel ContextOrg[n]to be currently filtered from among the original neighboring pixels andneighboring pixels ContextOrg[n−1] and ContextOrg[n+1] located at leftand right sides of the neighboring pixel ContextOrg[n] so as to generatea first filtered neighboring pixel. Outermost filtered neighboringpixels from among the first filtered neighboring pixels have values ofthe original neighboring pixels. That is,ContextFiltered1[0]=ContextOrg[0] andContextFiltered1[4N−1]=ContextOrg[4N−1].

Similarly, the neighboring pixel filtering unit 1210 may calculateweighted average values between the first filtered neighboring pixelsContextFiltered1[n] so as to generate second filtered neighboring pixelsContextFiltered2[n]. For example, the neighboring pixel filtering unit1210 generates the second filtered neighboring pixels by applying a3-tap filter to the first filtered neighboring pixelsContextFiltered1[n] as represented by Equation (5).ContextFiltered2[n](ContextFiltered1[n−1]+2*ContextFiltered1[n]+ContextFiltered1[n+1])/4  (5)

Referring to Equation (5), the neighboring pixel filtering unit 1210calculates a weighted average value of a neighboring pixelContextFiltered1[n] to be currently filtered from among the firstfiltered neighboring pixels and neighboring pixels ContextFiltered1[n−1]and ContextFiltered1[n+1] located at left and right sides of theneighboring pixel ContextFiltered1[n] so as to generate a secondfiltered neighboring pixel. Outermost filtered neighboring pixels fromamong the second filtered neighboring pixels have values of the firstneighboring pixels. That is, ContextFiltered2[0]=ContextFiltered1[0] andContextFiltered2[4N−1]=ContextFiltered1[4N−1]. The above-describedneighboring pixel filtering process may be repeated more than twice.Also, the number of taps of a filter for filtering neighboring pixels isnot limited to three as described above and may be variably changed.Also, the number of taps of a filter and coefficient of the filter forfiltering neighboring pixels can be adaptively applied.

The reference pixel determining unit 1220 determines the filteredneighboring pixels or the original neighboring pixels as referencepixels to be used to perform intra prediction on the current codingunit. In more detail, the reference pixel determining unit 1220 selectsthe original neighboring pixels, the first filtered neighboring pixels,or the second filtered neighboring pixels as the reference pixelsaccording to the size of the current coding unit and the type of anintra prediction mode to be currently performed. For example, if areference index of a prediction mode using the original neighboringpixels is 0, a reference index of a prediction mode using the firstfiltered neighboring pixels is 1, and a reference index of a predictionmode using the second filtered neighboring pixels is 2, the referencepixel determining unit 1220 may determine the type of neighboring pixelsto be used to perform intra prediction according to the size of thecurrent coding unit and the type of an intra prediction mode to becurrently performed, as shown in Table 2.

TABLE 2 Size of Coding Unit Prediction N × N Mode 16 × 32 × 64 × (N > 04 × 4 8 × 8 16 32 64 64) 1 0 1 0 0 0 0 2 0 1 0 0 0 0 3 0 1 0 0 0 0 4 0 10 0 0 0 5 1 2 2 2 2 2 6 1 2 2 2 — — 7 1 2 2 2 — — 8 1 2 2 2 — — 9 — — 22 — — 10 — — 2 2 — — 11 — — 2 2 — — 12 — — 2 2 — — 13 — — 2 2 — — 14 — —2 2 — — 15 — — 2 2 — — 16 — — 2 2 — — 17 — — 2 2 — — 18 — — 2 2 — — 19 —— 2 2 — — 20 — — 2 2 — — 21 — — 2 2 — — 22 — — 2 2 — — 23 — — 2 2 — — 24— — 2 2 — — 25 — — 2 2 — — 26 — — 2 2 — — 27 — — 2 2 — — 28 — — 2 2 — —29 — — 2 2 — — 30 — — 2 2 — — 31 — — 2 2 — — 32 — — 2 2 — —

Referring to Table 2, for example, if the current coding unit has a sizeof 32×32 and intra prediction is performed by using intra predictionmode 4, a reference index is 0 and thus the reference pixel determiningunit 1220 determines the original neighboring pixels ContextOrg[n] asthe reference pixel to be used to perform intra prediction on thecurrent coding unit. The intra prediction modes in Table 2 representintra prediction modes shown in Table 3. Also, “−” in Table 2 representsthat an intra prediction mode for a corresponding size of a coding unitis not defined. Table 2 is based on the intra prediction modes shown inTable 3, and is exemplarily shown. Unlike Table 3, as long as differentintra prediction modes are set according to the sizes of coding units,the reference indices in Table 2 may be differently set.

Referring back to FIG. 14, if the reference pixel determining unit 1220determines reference pixels to be used to perform intra prediction onthe current coding unit from among the original neighboring pixels andthe filtered neighboring pixels, the intra prediction performing unit1230 performs intra prediction by using the determined reference pixelsaccording to an intra prediction mode that is available according to thesize of the current coding unit, so as to generate a prediction codingunit.

FIG. 15 is a table showing the numbers of intra prediction modesaccording to the sizes of coding units, according to an exemplaryembodiment.

According to an exemplary embodiment, the number of intra predictionmodes to be applied to a coding unit (a decoding unit in a decodingprocess) may be variably set. For example, referring to FIG. 15, if thesize of a coding unit on which intra prediction is performed is N×N, thenumbers of intra prediction modes actually performed on 2×2, 4×4, 8×8,16×16, 32×32, 64×64, and 128×128-sized coding units may be respectivelyset as 5, 9, 9, 17, 33, 5, and 5 (in Example 2). For another example,when a size of a coding unit to be intra-predicted is N×N, numbers ofintra prediction modes to be actually performed on coding units havingsizes of 2×2, 4×4, 8×8, 16×16, 32×32, 64×64, and 128×128 may be set tobe 3, 17, 34, 34, 34, 5, and 5. The numbers of intra prediction modes tobe actually performed are differently set according to the sizes ofcoding units because overheads for encoding prediction mode informationdiffer according to the sizes of coding units. In other words, a smallcoding unit occupies a small portion of entire image data but may have alarge overhead in order to transmit additional information such asprediction mode information of the coding unit. Accordingly, if a smallcoding unit is encoded by using an excessively large number ofprediction modes, the number of bits may be increased and thuscompression efficiency may be reduced. Also, a large coding unit, e.g.,a coding unit equal to or greater than 64×64, generally corresponds to aplain region of image data, and thus encoding of the large coding unitby using an excessively large number of prediction modes may also reducecompression efficiency.

Thus, according to an exemplary embodiment, coding units are roughlyclassified into at least three sizes such as N1×N1 (where 2≦N1≦4, and N1is an integer), N2×N2 (where 8≦N2≦32, and N2 is an integer), and N3×N3(where 64≦N3, and N3 is an integer). If the number of intra predictionmodes performed on the coding units of N1×N1 is A1 (where A1 is apositive integer), the number of intra prediction modes performed on thecoding units of N2×N2 is A2 (where A2 is a positive integer), and thenumber of intra prediction modes performed on the coding units of N3×N3is A3 (where A3 is a positive integer), the numbers of intra predictionmodes performed according to the sizes of the coding units may be set tosatisfy A3≦A1≦A2. That is, if a current picture is split into smallcoding units, medium coding units, and large coding units, the mediumcoding units may be set to have the largest number of prediction modesand the small coding units and the large coding units may be set to havea relatively small number of prediction modes. However, the exemplaryembodiment is not limited thereto and the small and large coding unitsmay also be set to have a large number of prediction modes. The numbersof prediction modes according to the sizes of coding units in FIG. 15are exemplarily shown and may be changed.

FIG. 16A is a table showing intra prediction modes applied to a codingunit having a predetermined size, according to an exemplary embodiment.

Referring to FIGS. 15 and 16A, for example, when intra prediction isperformed on a coding unit having a 4×4 size, a vertical mode (mode 0),the coding unit may have a horizontal mode (mode 1), a direct current(DC) mode (mode 2), a diagonal down-left mode (mode 3), a diagonaldown-right mode (mode 4), a vertical-right mode (mode 5), ahorizontal-down mode (mode 6), a vertical-left mode (mode 7), and ahorizontal-up mode (mode 8).

FIG. 16B illustrates directions of the intra prediction modes shown inFIG. 16A. In FIG. 16B, numbers at ends of arrows represent predictionmodes corresponding to prediction directions indicated by the arrows.Here, mode 2 is a DC mode having no directivity and thus is not shown inFIG. 16B.

FIG. 16C is a diagram for describing a method of performing intraprediction on a coding unit by using the intra prediction modes shown inFIG. 16A.

Referring to FIG. 16C, a prediction coding unit is generated byperforming an available intra prediction mode determined according tothe size of a current coding unit by using neighboring pixels A throughM of the current coding unit. For example, an operation of performingprediction encoding on a current coding unit having a 4×4 size accordingto mode 0, i.e., a vertical mode, shown in FIG. 16A will be described.Initially, values of the neighboring pixels A through D at an upper sideof the current coding unit are predicted as pixel values of the currentcoding unit. That is, the value of the neighboring pixel A is predictedas a value of four pixels in a first column of the current coding unit,the value of the neighboring pixel B is predicted as a value of fourpixels in a second column of the current coding unit, the value of theneighboring pixel C is predicted as a value of four pixels in a thirdcolumn of the current coding unit, and the value of the neighboringpixel D is predicted as a value of four pixels in a fourth column of thecurrent coding unit. After that, the pixel values of the current codingunit predicted by using the neighboring pixels A through D aresubtracted from the pixel values of the original current coding unit soas to calculate an error value and then the error value is encoded.Meanwhile, when various intra prediction modes are applied, neighboringpixels used as reference pixels may be original neighboring pixels orfiltered neighboring pixels as described above.

FIG. 17 illustrates intra prediction modes applied to a coding unithaving a predetermined size, according to another exemplary embodiment.

Referring to FIGS. 15 and 17, for example, when intra prediction isperformed on a coding unit having a 2×2 size, the coding unit may havetotally five modes such as a vertical mode, a horizontal mode, a DCmode, a plane mode, and a diagonal down-right mode.

Meanwhile, if a coding unit having a 32×32 size has 33 intra predictionmodes as shown in FIG. 15, directions of the 33 intra prediction modesneed to be set. According to an exemplary embodiment, in order to setintra prediction mode having various directions in addition to the intraprediction modes illustrated in FIGS. 16A through 16C, and 17,prediction directions for selecting neighboring pixels used as referencepixels of pixels of the coding unit are set by using (dx, dy)parameters. For example, if each of the 33 prediction modes is definedas mode N (where N is an integer from 0 to 32), mode 0 may be set as avertical mode, mode 1 may be set as a horizontal mode, mode 2 may be setas a DC mode, mode 3 may be set as a plane mode, and each of mode 4through mode 31 may be defined as a prediction mode having a directivityof tan⁻¹(dy/dx) by using (dx, dy) represented as one of (1,−1), (1,1),(1,2), (2,1), (1,−2), (2,1), (1,−2), (2,−1), (2,−11), (5,−7), (10,−7),(11,3), (4,3), (1,11), (1,−1), (12,−3), (1,−11), (1,−7), (3,−10),(5,−6), (7,−6), (7,−4), (11,1), (6,1), (8,3), (5,3), (5,7), (2,7),(5,−7), and (4,−3) shown in Table 3.

TABLE 3 mode # dx dy mode 4 1 −1 mode 5 1 1 mode 6 1 2 mode 7 2 1 mode 81 −2 mode 9 2 −1 mode 10 2 −11 mode 11 5 −7 mode 12 10 −7 mode 13 11 3mode 14 4 3 mode 15 1 11 mode 16 1 −1 mode 17 12 −3 mode 18 1 −11 mode19 1 −7 mode 20 3 −10 mode 21 5 −6 mode 22 7 −6 mode 23 7 −4 mode 24 111 mode 25 6 1 mode 26 8 3 mode 27 5 3 mode 28 5 7 mode 29 2 7 mode 30 5−7 mode 31 4 −3 Mode 0 is a vertical mode, mode 1 is a horizontal mode,mode 2 is a DC mode, mode 3 is a plane mode, and mode 32 is a bi-linearmode.

FIGS. 18A through 18C are diagrams for describing intra prediction modeshaving various directivities, according to an exemplary embodiment.

As described above in relation to Table 3, intra prediction modesaccording to an exemplary embodiment may have various directivities oftan⁻¹(dy/dx) by using a plurality of (dx, dy) parameters.

Referring to FIG. 18A, neighboring pixels A and B on an extension line160 having an angle of tan⁻¹(dy/dx) according to (dx, dy) values inTable 3 with respect to a current pixel P in a current coding unit to bepredicted may be used as a predictor of the current pixel P. In thiscase, neighboring pixels used as a predictor may be previously encodedand restored pixels of a previous coding unit at upper and left sides ofa current coding unit. Also, if the extension line 160 passes betweentwo neighboring pixels positioned at integer locations, one of theneighboring pixels closer to the extension line 160 than the other maybe used as the predictor of the current pixel P.

Also, if the extension line 160 passes between two neighboring pixelspositioned at integer locations, one of the neighboring pixels closer tothe current pixel P than the other may be used as the predictor of thecurrent pixel P, or a weighted average value calculated in considerationof distances from the neighboring pixels to a crossing of the extensionline 160 and a line between the neighboring pixels may be used as thepredictor of the current pixel P.

FIGS. 18B and 18C are diagrams for describing a process of generating apredictor when the extension line 160 passes between two neighboringpixels positioned at integer locations, according to an exemplaryembodiment.

Referring to FIG. 18B, if the extension line 160 having an angle oftan⁻¹(dy/dx) to be determined according to a (dx, dy) value of each modepasses between neighboring pixels A 151 and B 152 positioned at integerlocations, as described above, one of the neighboring pixels A 151 and B152 closer to the extension line 160 or a weighted average valuecalculated in consideration of distances from the neighboring pixels A151 and B 152 to a crossing of the extension line 160 and a line betweenthe neighboring pixels A 151 and B 152 may be used as a predictor of thecurrent pixel P. For example, if the distance between the crossing andthe neighboring pixel A 151 is f and the distance between the crossingand the neighboring pixel B 152 is g, the predictor of the current pixelP may be obtained as (A*g+B*f)/(f+g). Here, f and g may be distancesregulated as integers. In actual software or hardware implementation,the predictor of the current pixel P may be obtained by performing ashift operation such as (g*A+f*B+2)>>2. As illustrated in FIG. 18B, ifthe extension line 160 passes a ¼ location between the neighboringpixels A 151 and B 152, which is closer to the neighboring pixel A 151,the predictor of the current pixel P may be obtained as (3*A+B)/4. Thisvalue may be obtained by performing a shift operation such as(3*A+B+2)>>2 in consideration of rounding off.

Meanwhile, if the extension line 160 passes between the neighboringpixels A 151 and B 152, the section between the neighboring pixels A 151and B 152 may be slit into a predetermined number of sections, and aweighted average value calculated in consideration of distances betweenthe crossing and the neighboring pixels A 151 and B 152 in each sectionmay be used as the predictor. For example, referring to FIG. 18C, thesection between the neighboring pixels A 151 and B 152 is split intofive sections P1 through P5, a representative weighted average valuecalculated in consideration of distances between the crossing and theneighboring pixels A 151 and B 152 in each section may be determined andmay be used as the predictor of the current pixel P. In more detail, ifthe extension line 160 passes section P1, a value of the neighboringpixel A 151 may be determined as the predictor of the current pixel P.If the extension line 160 passes section P2, a weighted average valuecalculated in consideration of distances between the center of sectionP2 and the neighboring pixels A 151 and B 152, i.e., (3*A+1*B+2)>>2, maybe determined as the predictor of the current pixel P. If the extensionline 160 passes section P3, a weighted average value calculated inconsideration of distances between the center of section P3 and theneighboring pixels A 151 and B 152, i.e., (2*A+2*B+2)>>2, may bedetermined as the predictor of the current pixel P. If the extensionline 160 passes section P4, a weighted average value calculated inconsideration of distances between the center of section P4 and theneighboring pixels A 151 and B 152, i.e., (1*A+3*B+2)>>2, may bedetermined as the predictor of the current pixel P. If the extensionline 160 passes section P5, a value of the neighboring pixel B 152 maybe determined as the predictor of the current pixel P.

Also, as illustrated in FIG. 18A, if the extension line 160 meets twoneighboring pixels such as the neighboring pixel A at an upper side andthe neighboring pixel B at a left side, an average value of theneighboring pixels A and B may be used as the predictor of the currentpixel P. Alternatively, the neighboring pixel A may be used if a valueof dx*dy is a positive number, and the neighboring pixel B may be usedif the value of dx*dy is a negative number. Also, neighboring pixelsused as reference pixels may be original neighboring pixels or filteredneighboring pixels as described above.

The intra prediction modes having various directivities in Table 3 maybe previously set at an encoder side and a decoder side, and thus eachcoding unit may transmit indices corresponding to only the set intraprediction modes.

According to an exemplary embodiment, as prediction encoding isperformed according to the intra prediction modes variably set accordingto the size of a coding unit, compression efficiency of an image may beimproved according to image characteristics. Also, according to anexemplary embodiment, as original neighboring pixels and filteredneighboring pixels are selectively used to perform intra prediction,prediction may be performed more variably and thus compressionefficiency of an image may be improved.

According to another exemplary embodiment, instead of using neighboringpixels previously determined according to the size of a current codingunit and the type of the intra prediction mode to be currentlyperformed, the intra prediction performing unit 1230 may performprediction on a current coding unit according to an available intraprediction mode by separately using original neighboring pixels, firstfiltered neighboring pixels, and second filtered neighboring pixels asreference pixels, and the reference pixel determining unit 1220 mayselect neighboring pixels having minimum costs as reference pixels to beultimately used to perform intra prediction on the current coding unit.

FIG. 21 is a flowchart illustrating a video encoding method according toan exemplary embodiment.

Referring to FIG. 21, in operation 1910, neighboring pixels of a currentcoding unit to be encoded are filtered to generate filtered neighboringpixels. As described above, the neighboring pixel filtering unit 1210filters neighboring pixels at upper and left sides of the current codingunit at least once so as to generate the filtered neighboring pixel.Here, a coding unit may be obtained by splitting a current picture basedon a maximum coding unit that is a coding unit having a maximum size,and a coded depth that is hierarchical split information of the maximumcoding unit.

In operation 1920, the filtered neighboring pixels or the originalneighboring pixels are selected as reference pixels to be used toperform intra prediction on the current coding unit. As described above,the reference pixel determining unit 1220 may select the referencepixels according to the size of the current coding unit and the type ofan intra prediction mode to be currently performed, as shown in Table 2.According to another exemplary embodiment, the reference pixeldetermining unit 1220 may compare resultant costs of intra predictionencoding performed by separately using the original neighboring pixeland the filtered neighboring pixels, and may determine the neighboringpixel to be ultimately used to perform intra prediction. Also, thereference pixel determining unit 1220 may signal to indicate whichneighboring pixel are selected among the original neighboring pixels andthe filtered neighboring pixels to perform intra prediction on thecurrent coding unit. In other words, intra prediction mode informationmay comprise reference index information indicating which neighboringpixel are selected among the original neighboring pixels and thefiltered neighboring pixels to perform intra prediction on the currentcoding unit. When the reference pixels to be used are preset at anencoding end and a decoding end as shown in Table 2, the reference indexinformation need not to be transmitted.

In operation 1930, intra prediction is performed on the current codingunit by using the selected reference pixels. As described above, theintra prediction performing unit 1230 generates a prediction coding unitby performing intra prediction on the current coding unit by applying anintra prediction mode that is available in the current coding unit byusing the selected reference pixels, and outputs the prediction codingunit.

FIG. 22 is a flowchart illustrating a video decoding method according toan exemplary embodiment.

Referring to FIG. 22, in operation 2010, neighboring pixels of a currentdecoding unit to be decoded are filtered to generate filteredneighboring pixels.

In operation 2020, information about an intra prediction mode applied tothe current decoding unit is extracted from a bitstream. The informationabout the intra prediction mode may include information about an intraprediction mode applied to the current decoding unit and informationabout a reference index representing whether original neighboring pixelsor filtered neighboring pixels are used as reference pixels. If, asshown in Table 1, the same type of reference pixels to be used accordingto the intra prediction mode and the size of the current decoding unitis set at an encoder side and a decoder side, the information about thereference index is not necessarily transmitted.

In operation 2030, the filtered neighboring pixels or the originalneighboring pixels are selected as reference pixels to be used toperform intra prediction on the current decoding unit. As describedabove, if the information about the reference index is additionallyincluded in the bitstream, the reference pixels are selected accordingto the extracted information about the reference index. If, as shown inTable 2, reference pixels may be determined based on the size and theintra prediction mode of the current decoding unit, the originalneighboring pixels or the filtered neighboring pixels to be used as thereference pixels may be determined based on the size and the intraprediction mode of the current decoding unit.

In operation 2040, intra prediction is performed on the current decodingunit by using the extracted information about the intra prediction modeand the selected reference pixels.

The present invention can also be embodied as computer readable code ona computer readable recording medium. The computer readable recordingmedium is any data storage device that can store data which can bethereafter read by a computer system. Examples of the computer readablerecording medium include read-only memory (ROM), random-access memory(RAM), CD-ROMs, magnetic tapes, floppy disks, and optical data storagedevices. The computer readable recording medium can also be distributedover network coupled computer systems so that the computer readable codeis stored and executed in a distributed fashion.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby one of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the invention as defined by the following claims. The exemplaryembodiments should be considered in a descriptive sense only and not forpurposes of limitation. Therefore, the scope of the invention is definednot by the detailed description of the invention but by the followingclaims, and all differences within the scope will be construed as beingincluded in the present invention.

What is claimed is:
 1. A method of decoding image, the methodcomprising: extracting information that indicates an intra predictionmode applied to a current block to be decoded, from a bitstream;determining one of non-filtered neighboring pixels adjacent to thecurrent block, first filtered neighboring pixels filtered by using theneighboring pixels according to a first filtering method, and secondfiltered neighboring pixels filtered by using the neighboring pixelsaccording to a second filtering method as reference pixels, based on atleast one of a size of the current block and an intra prediction mode ofthe current block; performing intra prediction on the current blockusing the extracted information and the determined reference pixels,wherein, when the current block has a size of N×N, where N is integer,the neighboring pixels include 2N neighboring pixels adjacent to anupper side and an upper right side of the current block and 2Nneighboring pixels adjacent to a left side and a below left side of thecurrent block, wherein the image is split into a plurality of maximumcoding units, according to information about a maximum size of a codingunit, the maximum coding unit is hierarchically split into one or morecoding units of depths according to split information, a coding unit ofa current depth is one of rectangular data units split from a codingunit of an upper depth, when the split information indicates a split forthe current depth, the coding unit of the current depth is split intocoding units of a lower depth, independently from neighboring codingunits, and the coding unit of the current depth is split into at leastone prediction unit.